Cylindrical bodies such as conduits or reservoirs which are subjected to internal pressure are often reinforced with strips of an elongate product, such as a wire, cable or ribbon, which are wound on to the bodies.
When the winding is effected under tension, a binding effect is achieved which enables the cylindrical body to be used at much higher internal pressures than the wall of the body could withstand by itself.
The binding can be done in rings, which may be contiguous or otherwise, each ring consisting of a plurality of superposed turns of the product wound around the tube with zero pitch, i.e. by several layers of the strip, e.g. metal ribbon, laid over one another and perhaps crossing over one another, each layer being would helically.
The elongate product is initially would on a supply spool, and, in order to apply it to the cylindrical body, the body may be rotated while the spool axis is fixed, or the spool may be rotated around the body, which is held fast. As the product is wound on to the body, it obviously unwinds from the spool, and to keep the strip under tension the unwinding of the spool must be braked.
It is important for the winding on to be carried out regularly, and especially important that the turns are at substantially the same tension. If not, after the tube is put into service there is a risk of the tension distributing itself among the windings, causing faults such as crossed wires. It is therefore useful to be able to control the tension in the product as it unwinds, to keep it at a substantially constant level.
When the unwinding of the spool is braked conventionally, by means of a braking member acting on the spool shaft, a disc brake, for example, it is difficult to control the tension, especially because of heating of the brake and because the braking torque varies with the diameter of the spool, i.e. with the amount of product left on the spool.